Pyrotechnic compositions for gas generation

ABSTRACT

A gas-generating pyrotechnic composition comprises: A. A FUEL SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AZIDES AND ALKALINE EARTH METAL AZIDES, B. AN ALKALI METAL OXIDANT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL PERCHLORATES AND BICHROMATES, C. A NITROGENOUS COMPOUND SELECTED FROM THE GROUP CONSISTING OF AMINOTETRAZOLE, AMINOTETRAZOLE HYDRATE, AZODICARBONAMIDE AND AZOTETRAZOLE, AND D. OPTIONALLY, SILICA AS AN ADDITIVE FOR REACTING WITH THE SOLID COMBUSTION RESIDUES OF THE COMPOSITION. Such compositions are particularly intended for the deployment of inflatable safety devices for driver and passenger protection in automobiles and other fast vehicles.

United States Patent [191 Doin et al,

[4 1 Oct. 14, 1975 PYROTECHNIC COMPOSITIONS FOR GAS GENERATION [73] Assignee: Societe Nationale des Poudres et Explosifs, Paris, France [22] Filed: Oct. 9, 1973 [21] Appl. No.: 404,762

[30] I Foreign Application Priority Data Oct. l7, l972 France 72.36738 [52] US. Cl. 149/35; 149/85; 149/2 [51] Int. Cl. C22C 37/00 [58] Field of Search 149/35, 85 2 [56] References Cited UNITED STATES PATENTS 3,785,674 l/l974 Poole et a1 149/35 X 3,797,854 3/1974 Poole et al l49/35 X Primary ExaminerStephen J. Lechert, Jr. Attorney, Agent, or FirmBucknam and Archer ABSTRACT A gas-generating pyrotechnic composition comprises:

a. a fuel selected from the group consisting of alkali metal azides and alkaline earth metal azides,

b. an alkali metal oxidant selected from the group consisting of alkali metal perchlorates and bichromates,

c. a nitrogenous compound selected from the group consisting of aminotetrazole, aminotetrazole hydrate, azodicarbonamide and azotetrazole, and

d. optionally, silica as an additive for reacting with the solid combustion residues of the composition.

Such compositions are particularly intended for the deployment of inflatable safety devices for driver and passenger protection in automobiles and other fast vehicles. I

12 Claims, N0 Drawings PYROTECHNIC COMPOSITIONS FOR GAS GENERATION The present invention is concerned with pyrotechnic compositions which generate combustion gases which are substantially non-toxic and which can be used, in particular, for the deployment of inflatable safety devices designed to protect the drivers and passengers of motor vehicles and other fast vehicles.

Propellant compositions 'are known containing, as an combustion catalyst, are also known. However, these compositions have only a limited yield of gas unless the mixture is so formulated as to provide a substantially stoichiometric quantity of oxygen which, in turn, leads to the production of toxic nitrogen oxides in the combustion gases. These propellant compositions are, however; primarily intended for the propulsion of rockets, for which purpose the production of such nitrogen oxides is not disadvantageous.

Gas-generating compositions have also been proposed which enable a large volume of non-toxic gas to be obtained in a very short period. These compositions comprise an inorganic oxidant and a plastics'binder and produce combustion gases containing a relatively high proportion of oxygen. The latter characteristic could be a disadvantage when such compositions are used for the deployment of inflatable safety devices in case of accidents in view of the risk of fire.

We have now developed a gas-generating pyrotechnic composition which gives, on combustion, a high yield of substantially non-toxic gas at a moderate temperature and within a very short period. The combustion gases obtained, predominantly consist of nitrogen, a non-reactive, gas, and have a low oxygen content and, in addition, thesolid combustion residues are physiologically harmless.

According to the present invention, we provide a gasgenerating pyrotechnic composition,,which comprises:

a. a fuel consisting of an alkali metal azide or alkaline earth metal azide,

b. an alkali metal oxidant selected from alkali meta perchlorates and bichromates,

c. a nitrogenous compound selected from aminotetrazole, aminotetrazole hydrate, azocarbonamide and azotetrazole, and

d. optionally, silica as an additive for reacting with;

the solid combustion residues of the composition.

In preferred compositions according to the invention, the weight ratio of azide: alkali metal oxidant 'is greater than 1 and the weight ratio nitrogenous compound:

azide is less than 48%.

The composition preferably comprises, by weight: a. 50 to parts of sodium azide, 1 b. 50 to 20 parts of potassium perchlorate, for a total of parts of azide and perchlorate,

I 65 c. up to 48%, with respect to the azide, of'amrnotetrazole or aminotetrazole hydrate, and d. at least 6%, with respect to the azide, of silica. The solid combustion residues of the composition according to the invention comprise alkali metal or alkaline earth metal oxides, most of which have an irritating physiological action and can cause burns, particularly when the constituents (a) and (b) are sodium azide and potassium perchlorate, respectively. The incorporation of silica in the composition enables these oxides to be suppressed by converting them into silicates which have no physiological action on the organism. For example, the total suppression of sodium oxide in the combustion residues is obtained by using 9.3% of silica with respect to the quantity of sodium azide; in practice, an amount of silica greater than 6% enables a partial, but satisfactory, suppression of sodium oxide to be obtained.

The presence of one or more of the specified nitrogenous compounds in the compositions according to the invention gives rise to the following advantages as compared with known gas-generating pyrotechnic compositions:

.i. the compositions according to the invention have a shorter ignition time, of the order of several milliseconds, while retaining a sufficiently high selfignition temperature for the composition to be completely safe in use;

ii. the yield of gas from the compositions according to the invention (expressed in litres per gram) is considerably greater and the combustion gases consist predominantly of nitrogen; and

iii. the potential expressed in cal/g, and the combustion temperature, are lower.

In addition, while the combustion gases obtained from the compositions according to the invention contain some carbon monoxide, the proportion of the latter can be kept below the critical toxic value and the combustion gases contain substantially less toxic nitrogen oxides than the gases obtained from known gasgenerating pyrotechnic compositions. Since the combustion gases obtained from the composition according to the invention are particularly intended for the deployment of inflatable safety devices, for which purpose the gases should be at only a moderate temperature, it is of particular importance to obtain:

i. the lowest possible combustion temperature, since the combustion gases have to be cooled before their introduction into the inflatable device,

ii. a maximum yield of gas and a very short ignition period, since for safety devices the time of inflation is usually required to be no longer than about 20 milliseconds, and

iii. a minimum toxicity, which requires a reduction in the amount of nitrogen oxides, the latter being much more dangerous than carbon monoxide.

In the case of compositions comprising sodium azide, potassium perchlorate, and aminotetrazole hydrate,

which has a potential of about 200 cal/g and which yields, on combustion, 0.5 litre of nitrogen per gram, together with water vapour and a small quantity of hydrogen, if it is desired to obtain combustion gases in mm. 30 ,the ratio NaNn should be 6 less than i.e. less than 8.5%

When a composition diagram is constructed in which the ordinates represent the weight ratio NaN /KCl and the abscissae the weight ratio aminotetrazole/- NaN the curve passing through the three points, a, b and c, defined as follows:

a b c ordinates NaM/KCIO 70/30 60/40 50/50 Aminotctrazole NaN 85% abscissae 27% 48% delimits a region situated below the curve, in which the amount of carbon monoxide in the combustion gases does not exceed about 100 ppm.

In addition, given that ignition difficulties are encountered when the ratio NaN /KCl0 is less than 1, the region representing the most preferred compositions is that lying between the curve defined by the three points a, b and c, the ordinate axis, and the straight line parallel to the abscissae axis for which the ordinate value 1, corresponding to equal amounts of potassium perchlorate and sodium azide.

In the following examples reference will be made, for the purpose of comparison, to three gas-generating pyrotechnic compositions illustrative of the prior art and containing only sodium azide, potassium perchlorate and silica. The formulation of these compositions, in parts by weight, was as follows:

(average particle size 30a).

The potentials obtained experimentally with these compositions decreased as the amount of perchlorate decreases, and were equal to: 1330 cal/g, 1270 cal/g and 1220 cal/g for compositions A, B and C respectively.

The self-ignition temperature of the three compsitions was 415C.

The gas yield was from 0.248 l/g and 0.356 l/g (the gases being at normal pressure and temperature) and the combustion gases contained from 96% to 99.5% of nitrogen mass of the combustion gases. The elimination of sodium oxide in the solid combustion residues was satisfactory due to the presence of powdered silica in the indicated quantities.

The use of other alkali metal azides or other alkaline earth metal azides and of other alkali metal perchlorates or bichromates gives different, but comparable, values and results. Compositions A. B, C show the importance of using large amounts of azide. since, on the one hand, the combustion temperature, which is related to the potential of the composition, decreases and, on the other hand, the yield of gases increases. but concommitantly the formation of nitrogen oxides also increases and, for example, in the range of proportions comprised between the compositions of the prior art type have a higher toxicity due to the excessive formation of nitrogen oxides.

The present invention is illustrated by the following examples in which all parts and percentages are by weight.

EXAMPLES Pyrotechnic compositions having the following formulations were prepared and formed into pellets:

Composition D E F G NaN 60 60 5O 50 KCIO 40 40 50 S0 aminotetrazole l6 18 24 28 Toxicity of the combustion gases expressed in parts per million Carbon monoxide 50 l 100 90 2l00 Nitrogen oxides 80 30 I I0 45 The potentials and combustion rates of these compositions were lower than those of compositions A, B and C having the same NaN /KC10 ratio. The potentials obtained were about the order of 1000 cal/g and the gaseous yields were about 0.45 l/g (at normal pressure and temperature).

The use of other specified nitrogenous compounds, that is aminotetrazole hydrate, azodicarbonamide and azotetrazole, gives different, but comparable, values and the effects observed are similar.

By way of comparison, there are given below the combustion characteristics of two compositions comprising only sodium azide and potassium perchlorate.

Composition ll NaN; 50 parts KCIO 50 parts Aluminum stearate 3 parts (necessary for formation of pellets) Characteristics .Gas yield 0.288 l/g Combustion rate 80 mm/s at 34 bars Composition l "NaN parts KCIO 30 parts Aluminum stearate 3 parts Characteristics Gas yield 0.356 l/g Gases produced 0, 0.8%

Combustion rate 350 mm/s at bars.

The following example illustrates the employment of aminotetrazole and silica in a gas-generating pyrotechnic composition according to the invention.

Composition K (in the form of pellets) NaN 70 parts KClO, 30 parts aminotetrazole 6.5 parts SiO (average particle size 30y) 6.5 parts The combustion characteristics obtained with this composition were as follows:

Solid residues In the foregoing examples, the preparation of the pyrotechnic compositions according to the invention is effected in a conventional manner by mixing of the constituents. Shaped charges can be formed by incorporating a suitable binder and subjecting the composition to a conventional pelleting or extrusion operation. Various shapes can be used according to the firing characteristics required. By way of example, compositions containing from 1 to 2 parts by weight of a metal stearate and in the form of pellets of 9.5 mm diameter and 3 mm thickness, are suitable.

The constituents are preferably physically treated to select a particular particle size fraction and/or to eliminate moisture. For example, potassium perchlorate is preferably dried at 80C for 12 hours and sodium azide is preferably crushed by rollers to pass through a 295 p. screen. The mixing should be carried out to obtain a mixture which is powdery and apparently homogeneous. Simultaneous introduction of the constituents into a mixer, such as a V mixer (Moritz), and mixing for 1 hour is suitable in most cases.

When pellets are required, pelletising of the powder mixture is preferably effected under the following working conditions in, for example a pelletiser of the Stokes type:

Pelleting pressure: 2 to l tonnes/cm Moisture content: 30% to 40% Temperature: 21 to 29C Binder: aluminum stearate, polytetrafluoroethylene, or potassium bromide.

We claim: 1. A gas-generating pyrotechnic composition, which comprises:

a. a fuel selected from the group consisting of alkali metal azides and alkaline earth metal azides,

b. an alkali metal oxidant selected from the group consisting of alkali metal perchlorates and bichromates,

c. a nitrogenous compound selected from the group consisting of aminotetrazole, aminotetrazole hydrate, azodicarbonamide and azotetrazole.

2. The composition according to claim 1 additionally comprising silica.

3. A composition as set forth in claim 1, wherein the weight ratio of azide: alkali metal oxidant is greater than 1.

4. A composition as set forth in claim 1, wherein the weight ratio of nitrogenous compound: azide is less than 48%.

5. A composition as set forth in claim 2, which comprises:

a. sodium azide,

b. potassium perchlorate,

c. a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate, and

d. silica.

6. A composition as set forth in claim 2, which comprises, by weight:

a. 50 to parts of sodium azide,

b. 50 to 20 parts of potassium perchlorate, for a total of parts of azide and perchlorate,

0. up to 48%, with respect to the azide, of a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate, and

d. at least 6%, with respect to the azide, of silica.

7. A composition as set forth in claim 6, which comprises, by weight, about 9.3% of silica with respect to the azide.

8. A composition as set forth in claim 6, which comprises, by weight:

a. 50 to 60 parts of sodium azide,

b. 50 to 40 parts of potassium perchlorate, for a total of 100 parts of azide and perchlorate, and

c. 28 to 48%, with respect to the azide, of a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate.

9. A composition as set forth in claim 6, which comprises, by weight:

a. about 70 parts of sodium azide,

b. about 30 parts of potassium perchlorate,

c. about 6.5 parts of a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate, ahd

d. about 6.5 parts of silica.

10. A composition as set forth in claim 5, in which- NaN;, 50 aminotetrazole 24 ii. the straight line parallel to the abscissae axis for which the ordinate value 1, and

iii. the axis of the ordinates.

11. A composition as set forth in claim 2, in the form of pellets.

12. A composition as set forth in claim 11, wherein said pellets comprise a binder selected from the group consisting of aluminium stearate, polytetrafluoroethylene and potassium bromide. 

1. A GAS-GENERATING PYROTECHNIC COMPOSITION, WHICH COMPRISES: A. A FUEL SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL AZIDES AND ALKALINE EARTH METAL AZIDES, B. AN ALKALI METAL OXIDANT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL PERCHLORATES AND BICHROMATES, C. A NITROGENOUS COMPOUND SELECTED FROM THE GROUP CONSISTING OF AMINOTETRAZOLE, AMINOTETRAZOLE HYDRATE, AZODICARBONAMIDE AND AZOTETRAZOLE.
 2. The composition according to claim 1 additionally comprising silica.
 3. A composition as set forth in claim 1, wherein the weight ratio of azide: alkali metal oxidant is greater than
 1. 4. A composition as set forth in claim 1, wherein the weight ratio of nitrogenous compound: azide is less than 48%.
 5. A composition as set forth in claim 2, which comprises: a. sodium azide, b. potassium perchlorate, c. a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate, and d. silica.
 6. A composition as set forth in claim 2, which comprises, by weight: a. 50 to 80 parts of sodium azide, b. 50 to 20 parts of potassium perchlorate, for a total of 100 parts of azide and perchlorate, c. up to 48%, with respect to the azide, of a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate, and d. at least 6%, with respect to the azide, of silica.
 7. A composition as set forth in claim 6, which comprises, by weight, about 9.3% of silica with respect to the azide.
 8. A composition as set forth in claim 6, which comprises, by weight: a. 50 to 60 parts of sodium azide, b. 50 to 40 parts of potassium perchlorate, for a total of 100 parts of azide and perchlorate, and c. 28 to 48%, with respect to the azide, of a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate.
 9. A composition as set forth in claim 6, which comprises, by weight: a. about 70 parts of sodium azide, b. about 30 parts of potassium perchlorate, c. about 6.5 parts of a nitrogenous compound selected from the group consisting of aminotetrazole and aminotetrazole hydrate, and d. about 6.5 parts of silica.
 10. A composition as set forth in claim 5, in which the weight ratios NaN3/KC104 and aminotetrazole/NaN3 fall within the region on a composition diagram in which the ratio NaN3/KC104 is plotted as the ordinates and the ratio aminotetrazole/NaN3 as the abscissae, defined by i. the curve passing through three points, a, b and c, which are respectively defined as follows:
 11. A composition as set forth in claim 2, in the form of pellets.
 12. A composition as set forth in claim 11, wherein said pellets comprise a binder selected from the group consisting of aluminium stearate, polytetrafluoroethylene and potassium bromide. 